A strain gauge is a device for measuring dimensional change primarily on the surface of a specimen as the latter is subjected to mechanical, thermal, or a combination of both stresses. One type of strain gauge is attached to the specimen surface and amplifies mechanically the surface distortion so that the change can be measured on a simple dial indicator. Other types of strain gauges measure the displacement of light rays through an optical system that is actuated by the surface strain, or convert this strain into an electrical signal. The mechanical, electro-mechanical, and optical strain gauge devices are considered extensometers, and their use generally is limited to materials properties testing, or as calibration tools.
The electrical type of strain gauge is in wide use today and has found applications far beyond those of a conventional extensometer. Electrical-type strain gauges may be based upon the measurement of a capacitance, an inductance, or a resistance change that is proportional to strain.
The principle of a resistance-type strain gauge can be illustrated with a conductor in rod shape. As the rod is elongated in response to tensile stress, the length of the rod increases and its cross-sectional area decreases to produce a resistance increase when the basic resistivity of the material remains reasonably constant. The resistance change, .DELTA.R/R, is related to the length change, .DELTA.L/L, or strain, by the strain sensitivity or gauge factor.
Most commercially available strain gauges are rigid structures based upon metals or semiconductor materials. Metallic strain gauges typically have gauge factors in the range of about 2.0 to 4.5, whereas semiconductor-type strain gauges may have gauge factors as high as 150.
Such strain gauges are adequate for measuring strain in rigid structures, such as bridges, buildings, machine parts, etc. There are certain applications, nevertheless, for which such rigid strain gauges are inadequate. For example, such rigid gauges have proven to be inadequate for measuring strains in biological tissues, such as ligament strains. Typical rigid strain gauges fail when they are subjected to strains of greater than about 2 percent, but loaded ligaments are estimated to be capable of strains of 30 percent before reaching their yield point. See Kennedy, J. C. et al., J. Bone & Joint Surg. 58-A(3), 350-355, 1976. In addition, the rigidity of metal and semiconductor gauges is so great that such gauges could alter the properties of any soft tissue onto which they are bonded. Thus, there is a need for what might be referred to as soft strain gauges.